It is a known technique to remove flash from molded plastic and elastomeric articles and paint or coatings from various articles by contacting these with a chilling medium, generally at cryogenic temperature, to embrittle the flash or coating, and subjecting such articles to impact by a high velocity stream of solid particles in the form of shot or pellets.
In a typical operation the piece or pieces to be treated are introduced into a heat-insulated chamber maintained at required low temperature and the stream of blasting media is impelled at high velocity against the surface of each piece by a rotating impeller or so-called throwing wheel. The discharged blasting media together with the fragments of the flash or coating thereby removed, are collected and conveyed out of the treating chamber to a screening apparatus in which the blasting media is separated and recovered for recycling to the blasting operation, and the larger fragments of flash and coating materials as well as fines are discharged. A system of this general type is described in U.S. Pat. No. 3,824,739.
In certain of these systems, as shown for example in Canadian Pat. No. 1,112,048, the used media after impacting the article being treated, together with the refuse comprising material removed from the article by the impact, falls to the bottom of the treating chamber. The material from the bottom of the chamber is conveyed by a screw conveyor to a reservoir. From the reservoir, the collected media and refuse are removed by a flexible helical conveyor and transported to a separation apparatus provided with screens of graduated size, whereby the clean blasting media freed of refuse is recovered for recirculation to the throwing wheel.
In systems employed for the removal of organic coatings from articles by shot blasting the chilled articles, these coatings fall off in discrete pieces that vary in size from larger flakes to fine dust which become mixed with the used shot. In order to reuse the shot, the mixture falling to the bottom of the treating chamber needs to be collected and transported to the separation system. Since the coating removal process will operate from room temperature to about -200.degree. F. (-129.degree. C.), the media collection system must be capable of withstanding the thermal contraction and expansion encountered in this temperature range. In addition, moisture will accumulate in the system due to condensation, requiring the mechanism to be resistant to water freezing and accumulation of ice. The treating chamber is relatively large and comprises a correspondingly large floor that needs to be freed of the mixture of refuse and shot falling thereon. Moreover, since the system is generally cooled by a liquefied gas, such as liquefied nitrogen, the treating chamber must be thermally insulated to minimize consumption of the coolant. Accordingly, for economic operation, the system, including the arrangement for collection and discharge of the mixture of refuse and shot at the bottom of the treating chamber, must be designed to minimize heat leak into the chamber and must be free of openings which would permit escape of the liquefied gas, or the entry of water vapor into the chamber.
A recently advocated system employed in an attempt to overcome the foregoing problems associated with a cryogenic system for shot blasting of coated articles, made use of the combination of a drag conveyor and a cross screw conveyor. The drag conveyor comprised a series of flat bars moved along the bottom of an insulated chamber by endless chains attached to sprockets mounted on a minimum of two rotating shafts penetrating the insulated chamber. These bars push the mixture of blasting media and removed coatings toward the cross screw conveyor, which latter is a helical auger turning in a trough. The helical auger is also mounted on a shaft which penetrates the chamber. As the helical auger rotates, it moves the mixture toward an opening at one end of the trough for transfer to the separation system.
The described system of a drag conveyor and cross screw conveyor has a number of disadvantages. The drag conveyor uses a roller chain that is subject to failure from the repeated freezing of moisture trapped between the side bars of the chain. Since the operating temperature of the chamber is lower than the rated temperature for known lubricants, the bearings for the shafts on which the sprockets are mounted, must be located outside of the chamber. Thus, the shafts and their openings into the chamber wall result in a large heat leak into the system. Also, since the drag conveyor bearings are mounted outside the chamber, an adjusting device must be provided to compensate for the thermal contraction of the roller chain. The drag conveyor and cross screw auger are complex devices that make it difficult to clean the bottom surface of the chamber. Further, large pieces of removed coating material or ice can easily jam these mechanisms. Tacky materials, such as uncured coatings, will cause severe problems from jamming due to the progressive build-up of layers on the operating components. Also, the uncured material may polymerize or solidify in inaccessible cracks and crevices.
Among the objects of the present invention is to provide an improved arrangement for the collection and discharge of the mixture of blasting media and refuse at the bottom of an insulated shot-blasting enclosure, avoiding the deficiencies encountered in operation of prior art systems.